Fusion splicers for optical fibers are described in “FITEL (registered trademark) Fusion Splicers & Tools Catalog”, Furukawa Electric, Volume 4, September 2010 (NPL 1). Two optical fibers are connected to each other by butting ends the optical fibers against each other and heat-fusing the ends together. The sensitivity of optical coherence tomography (OCT) is described in Michael A. Choma, “Sensitivity advantage of swept source and Fourier domain optical coherence tomography, Opt. Express 11, 2183-2189 (2003)” (NPL 2).